fix(perf): count a stall even when the scroll state went missing across it

On hdpi the stall watchdog logged a 1.9s stall during the hourly sports
refresh that the soak report never had: its worst gap was 655ms. The frame
that ended the stall was recorded as static, so its interval was dropped.

"Scrolling" is DisplayManager's scroll state at the moment a frame is
presented, and it goes missing mid-scroll: it expires after 2s without
activity, and any thread can clear it. Plugins call
set_scrolling_state(False) from their own display() (news, stocks, the odds
ticker's fallback), and Vegas captures some of those on the render thread
between two of its own frames. Vegas sets the state again only after its
next frame, so that frame is recorded as static -- along with the capture
or stall it followed.

One static frame between two scrolling frames, with the scroll resuming
within RESUME_SECONDS (1s), is now a frame of the scroll and both of its
intervals count, the first at the scroll's own hold (clearing the state
drops the hold to 1 too). Two static frames in a row still end the scroll.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/common/frame_timing.py

@@ -19,6 +19,19 @@ Only intervals between two consecutive *scrolling* frames count: a static
 screen that changes once a second has no timing to get wrong, and the first
 frame of a scroll has no predecessor worth measuring against.
 
+"Scrolling" is DisplayManager's scroll state when the frame is presented, and
+that state can go missing in the middle of a scroll. It expires after 2s
+without scroll activity, which a long enough stall outlasts, and any thread can
+clear it: plugins call ``set_scrolling_state(False)`` from their own
+``display()``, and Vegas captures some of those on the render thread between
+two of its frames. The frame after that is recorded as static, and the interval
+it ends -- the stall, or the capture -- would vanish from the report. So a
+single static frame between two scrolling ones, with the scroll picking up
+again within ``RESUME_SECONDS``, is treated as a frame of the scroll: both of
+its intervals count. A second static frame in a row means the scroll really
+ended. (On hdpi on 2026-09-24 the watchdog logged a 1.9s stall that the soak
+report did not have; this is how.)
+
 A frame held for ``hold`` refreshes should arrive ``hold`` refresh periods
 after the one before it. One that arrives a whole refresh or more after that is
 **late**: the panel showed the previous frame again, which on a moving strip is
@@ -94,12 +107,16 @@ BUCKET_COUNT = 256
 #: See the module docstring.
 FREEZE_SECONDS = 0.25
 
-#: Two frames that are both "scrolling" can be at most DisplayManager's
-#: scroll_inactivity_threshold (2s) apart: after that the second is recorded
-#: as static. This used to be 1s, which silently dropped every 1-2s stall
-#: inside a scroll. It is now only a sanity bound.
+#: Intervals this long are not frames of one scroll. This used to be 1s,
+#: which silently dropped every 1-2s stall inside a scroll. It is now only a
+#: sanity bound.
 GAP_SECONDS = 5.0
 
+#: A frame recorded as static between two scrolling frames is a frame of the
+#: scroll whose state went missing, if the scroll resumes within this long.
+#: See "What is counted".
+RESUME_SECONDS = 1.0
+
 #: Buckets for freeze length, as cumulative counters a soak can difference.
 FREEZE_BUCKETS = ((0.5, "<0.5s"), (1.0, "0.5-1s"), (2.0, "1-2s"),
                   (float("inf"), "2s+"))
@@ -232,6 +249,9 @@ class FrameTimingRecorder:
         self._pending: List[Tuple[float, float, float, int]] = []
         self._static_frames = 0
         self._previous: Optional[Tuple[float, bool, int]] = None
+        # The interval ended by a static frame that followed a scrolling one,
+        # until the next frame shows whether the scroll went on.
+        self._unsure: Optional[Tuple[float, float, float, int]] = None
         self._last_flush: Optional[float] = None
         self._queue: "queue.SimpleQueue" = queue.SimpleQueue()
         self._worker: Optional[threading.Thread] = None
@@ -284,13 +304,28 @@ class FrameTimingRecorder:
         self.last_frame = (presented_at, scrolling, threading.get_ident())
         if not scrolling:
             self._static_frames += 1
+            # The scroll ended, or its state went missing for this frame: the
+            # next frame says which. Its hold may have been dropped with the
+            # state, so the interval is due at the scroll's own.
+            self._unsure = None
+            if previous is not None and previous[1]:
+                self._unsure = (presented_at - previous[0], blit, wait, previous[2])
         elif self.watchdog is None and self.scrolling_now is not None \
                 and os.environ.get("LEDMATRIX_STALL_WATCHDOG", "1") != "0":
             self.watchdog = StallWatchdog(self, **watchdog_settings())
             self.watchdog.start()
-        elif previous is not None and previous[1]:
+        elif previous is not None:
             interval = presented_at - previous[0]
-            if interval < GAP_SECONDS:
+            unsure, self._unsure = self._unsure, None
+            if previous[1]:
+                if interval < GAP_SECONDS:
+                    self._pending.append((interval, blit, wait, hold))
+            elif unsure is not None and interval < RESUME_SECONDS:
+                # One static frame between two scrolling ones: the scroll never
+                # stopped, only its state did. Both intervals were motion.
+                self._static_frames -= 1
+                if unsure[0] < GAP_SECONDS:
+                    self._pending.append(unsure)
                 self._pending.append((interval, blit, wait, hold))
 
         if self._last_flush is None: